Social Networking

I'm in grad school and am in a class that is all about Web 2.0. We are studying social networks this week. It has really got me thinking about social networking and schools, which was the point :). I can see the benefits and pit falls of using social networking in a classroom. I was wondering if anyone on here has incorporated social networks into their classroom. If so how did it work? Good Bad. Even if you haven't tried do you think that social networks in schools should be limited to teachers so that we can collaborate with many different teachers?